Iodine Cell

• Introduction
• Temperature Control
• Setting of Servo Parameters

Introduction

The purpose of the Iodine cell is to look at long term drift of red shift of observed objects. The Iodine Cell consists of a cylindrical glass vessel filled with iodine which is heated by a heater/temperature controller to 50 Deg C.

Temperature Control

The temperature controller consists of 10inchx4inch silicon heating mat, Cole-Parmer cat. No. E-03125-40 and a Cole-Parmer model 89000-05 temperature controller. Since the heating mat is rated at 115VAC and the controller output is 240VAC, then an external resistor was connected in serial with the heater mat to reduce the voltage drive.

The controller setting is as follows

Parameter	Setting	Parameter Description
Thermocouple	Type J
Temp Scale	Celsius
Alarm setpoints	on
Mode	HI Alarm	Alarm Mode
Alarm SP	HI 75 DegC	Alarm setpoint
Alarm Hysteresis	2.0 DegC	determines when alarm is going to be out of alarm condition
Audible Alarm	on
Sensor Offset	+_ 00.0 at 000.0 DegC	Calibration feature
Over Temp Stop	10 DegC above SP	If SP + over temp > PV, then temperature controller will stop
Loop break stop	000.0 minutes	safety feature to stop control if output is on for specified time and PV temperature does not increase by more than 1 DegC
Control Action	Heat
Control Mode	PID
Auto Tune	Disable	enable/disable auto tune button
Proportional Band	32 DegC
Integral Time	378 seconds	correct for droop
Derivative Rate	90 seconds	reduces overshoot
Cycle Time	1 seconds	rate at which output is cycled
Run Time	Continuous	sets temperature controller operating time
Power Up Control	Last State	specify one of two conditions at turn on

Refer to operating manual on how to set parameters.

Setting of Servo Parameters

Low Value External Resistor 62 OHM

First attempt at setting PID parameters was to measure a step response. From the step response , the slope and delay were calculated

Maximum slope, R = 0.75 degC/sec

Delay, Td = 20 secs

and from these the PID parameters

Proportional Gain = RxTd / 0.9 = 16 DegC

Integral Gain = 3 x Td = 60 secs

Derivative Gain = 0 secs

Proportional gain of 16 was found to have too much overshoot and was reduced to 10 degC. This controlled the temperature to 50 degC +/- 0.2 degC.

The next technique uses the autotune feature of the controller. Upon completion, the controller set the following servo parameters

Proportional Gain = 32 DegC

Integral Gain = 378 secs

Derivative Gain = 90 secs

cycle time = 1 secs

The temperature was set to 70 degC and the following warm up transient response was measured

Time (secs)	Temperature (degC)
0	36.5
10	36.9
20	39.7
30	42.0
40	44.4
50	46.8
60	48.7
70	50.5
80	52.2
90	53.7
100	55.2
110	56.5
120	58.0
130	59.3
140	60.6
150	61.6
160	62.6
170	63.3
180	64.2
190	64.9
200	65.7
210	66.6
220	67.3
230	68.0
240	68.8
260	70.2
270	71.0
280	71.6
290	72.2
300	72.7
360	74.3
420	75.0
480	75.2
540	74.6
600	73.8
660	73.8
720	73.0
780	72.4
840	72.2
900	71.4
960	71.1
1040	71.2

Plot of 62 ohm transient response to 70 degC.

Larger External Resistor 480OHM

A larger external resistor, 480 ohm, was used to reduce the heating ability of silicon mat. The autotune feature of the controller was used, which set the following servo parameters

Proportional Gain = 8 DegC

Integral Gain = 274 secs

Derivative Gain = 65 secs

cycle time = 1 secs

The temperature was set to 60 degC and the following warm up transient response was measured

Time (secs)	Temperature (degC)
0	34.5
60	38.7
120	42.9
180	46.7
240	49.2
300	51.3
360	52.9
420	54.3
480	55.7
540	56.9
600	58.1
660	58.9
720	59.5
780	60.2
840	60.7
900	60.8
960	60.8
1040	60.8

Plot of 480 ohm transient response to 60 degC.

The heater was turned off and the following transient response was measured

Time (secs)	Temperature (degC)
0	56.2
60	54.3
120	52.4
180	50.3
240	48.5
300	46.0
360	45.6
420	44.4
480	43.1
540	41.9
600	40.8
660	39.8
720	38.8
780	38.0
840	37.1
900	36.3

Plot of 480 ohm transient response cooldown